Separation of gases by diffusion



June 30, 1959 G. T. KOHMAN ET AL 2,392,508

SEPARATION OF GASES BY DIFFUSION Filed April 17, 1957 SOURCE or 4 zxmusrMIXTURE PUMP OFGASES AND UNDER V A sroma: PRESSURE rA/v/r 42) 4a) SI'AGEsum: 2 sue: a

G. 7? KOHMAN IN VE N 7' 0195 W I? MASON k. B. MCAFEE-JR ATTORNEY UnitedStates Patent SEPARATION OF GASES BY DIFFUSION Girard T. Kohman, Summit,Warren P. Mason, West Orange, and Kenneth B. McAfee, Jr., Summit, NJ.,assignors to Bell Telephone Laboratories, Incorporated, New York, N.Y.,a corporation of New York f Application April 11, 1957, Serial No.653,471 5 Claims. c1. 183-2) This invention relates to methods andapparatus for the separation or purification of gases. Moreparticularly, it relates to methods and apparatus for the separation orpurification of gases by diffusion and/ or by selective diffusion.

It has long been known, for example, that helium will pass by diffusionat room temperature through glass and that an appreciable number ofother gases will more or less 'readily diffuse through various types ofglass, particularly at temperatures several hundred degrees centi gradeabove room temperature and through the more open types of glass andceramic materials having vitric properties, that is, ceramics whichinclude an appreciable amount of glass forming substances which act as abinder or network for holding the ceramic together. As a general rule,the rate of diffusion at a given temperature is approximatelyproportional to the pressure of the gas upon the glass or ceramic.

The present invention is based upon the discovery that'glasses andvitric ceramics subjected to tensile stress exceeding approximatelyfifty percent of the tensile stress at :which they fracture will exhibita substantially increased rate of diffusion over that which would beexpected on the basis of the heretofore accepted teachings of the priorart. The rate of diffusion increases still more rapidly as the tensionis further increased to more nearly approach the breaking tensilestress. The phenomenon has been found to be reversible, the glass orceramic recovering its normal properties when the tension is released.By way of a specific example of the phenomenon, the diffusion of heliumthrough chemical Pyrex glass at room temperature was increased by afactor of approximately ten when the glass was stressed to approximatelyninety percent of its breaking tensile strength as compared with thediffusion which would normally be expected on the basis of the teachingsof the prior art. The breaking tensile stress of a one inch diametersphere of chemical Pyrex glass .005 inch thick was, for example, foundto vary appreciably from sample to sample, but was usually between 100and 150 pounds per square inch, Flame annealing at approximately fivehundred degrees centigrade for a minute or two has been found todecrease the tendency of the glass to break under tension.

It is, of course, well known to those skilled in the art that whendiffusion is present at room temperature for any specific gas through aspecific type of glass or ceramic, the diffusion is usually increasedvery appreciably with increase in temperature.

Some gases, such as hydrogen and deuterium, will not diifuse through,for example, pure amorphous silica glass at room temperature to anysignificant extent but will diffuse in appreciable amounts attemperatures in the order of six to eight hundred degrees centigrade.

For glass which it is feasible to blow, a convenient diffusing elementis a sphere as shown in the accompanying drawings. Unsintered amorphoussilica, commonly known as thirsty glass, however, cannot readily beshaped into a sphere so that a diifusing element employing thirsty glassmust be designed to comprise substantially fiat difiusing surfaces, forexample, it can be a cubic or rectangular enclosure, one or more majorsurfaces of which consist of thin sheets of thirsty glass. In

general, a sphere is preferable since a'substantially uni form tensingof its entire surface is readily effected by the pressure of the gaseswithin it. However, reasonably satisfactory results can be obtained witha'substantially cubic enclosure or with a cylindrical enclosure.-

In some instances two gases which'it is desired to separate may bothpass by diffusion through a particular type of glass at a particulartemperature but at largely differing rates, in which case severalconsecutive stages of separation by diffusion will, obviously, tend toappreciably reduce or even to substantially eliminate the gas having theslower rate of diffusion, thus -purifying" the other. v I

A principal object of the invention is to facilitate a more rapidseparation or purification of gases by the phenomenon of diffusionthrough glass and ceramics."

Other and further objects, features and advantages of the invention willbecome more readily apparent during the course of the following detaileddescription of illustrative embodiments for practicing variousprinciples of the invention and from the appended claims. In theaccompanying drawings:

Fig. 1 illustrates diagrammatically an arrangement for practicingcertain principles of the invention; and

Fig. 2 illustrates diagrammatically a further arrangement for practicingprinciples of the invention.

In Fig. 1, a source 10 of a mixture of gases under presthe art so thatthe enhanced rate of diffusion will be continuously maintained.

To simplify the drawing, a single glass sphere 14 is shown in Fig. 1. Itis obvious that increased capacity can be readily realized by employinga plurality of spheres such as sphere 14 of Fig. 1, all connected tosource 10 and all enclosed within the gas tight enclosure 12. In atypical case, sphere 14 had a diameter of approximately one inch and thethickness of the glass was approximately .005 inch. The practicalconsiderations influencing the choice of the number vof spheres, thesphere diameters, and glass thickness are, of course, the volume of gas.per unit of time it is desired to purify by the dilfusion process andthe combination of maximum sphere diame eter and the minimum glassthickness which will withstand the maximum pressure it is feasible touse. The sphere 14 (or a plurality of such spheres all connected tosource 10, if several spheres are used) may conveniently be referred toas a diffusing section or simply as a diffuser."

The glass should, of course, be free from pinholes,

bubbles, rib lines, inclusions of foreign material and thin spots, andshould be flame annealed as described hereinabove, so that it canwithstand as great a tensile stress as possible, since the rate ofdiffusion rapidly increases with increase of tensile stress beyond theminimum stress at which the abnormal diffusion of the invention is firstrealized. I

As mentioned hereinabove, cubioor cylindrical diffusing enclosures canproduce satisfactory results: but a spherical shape is preferable in"that it is readily sus ordess readily =through;the-glass sphere 14 and asecond gasavvhich .willpnormallymot diffuse through the glass sphere 14t any appreciable, extent, it will be found that whenitheapressurewithin sphere 14 has been increased to .a point 'at-,which.,the tensilestress in the glass of sphere 14-exceeds fifty percent of the breakingtensile stress :of-the glass, (but is, of course, still less than thebreaking .tensile stress) the first gas will diffuse through thezgla sata,-an.-,appreciably greater rate than would be expected fromprior; artteachings and the second gas still not' diifusegthroughhthe.glass to anyappreciable extent.

As.=. a;specific.;example,- fora glass sphere of chemical Pyrexrglass;and a mixture of the two gases helium and hydrogen, at-room.temperature,the diffusion of helium through: the .glass ;=will1 be 3 verysubstantially increased when the-(glass isastressedbeyond fifty percentof its breaking: tensile strengthbut not to fracture of the glass,whereas the diffusion of hydrogen will still be substantiallynonexistent Thus substantially pure helium can be obtainedat almuch,faster-,ratethan would be expected from .rprior part teachings.

Detailed calculations. basedon the postulated structure.-of;;glass-..-and-measurements made of the heretoforedescrihedstress.enhanced dilfusion have led to the conclu'sionrthatl.glass; and ceramics must have submicroscopic fissureswhich are furtheropened by sutlicient tension to permit the helium or other diffusing gasto passgmoreireely. but which are nevertheless so small that thepassage;;or-ditfusion;.ofthe hydrogen or other.

nondifiusinggasiis' not; perceptibly aliected.

Sphere 14:;isi:surrounded :by a gas-tight. enclosure 12.

whichn'sconnected by 'tube'2t .tov an exhaust pump and storage tank :16.into. the tankof whichthe gas diifusing through;sphere.14 is pumped fromenclosure 12.. Where several-stages are to :beemployedin sequence toeffect a more: complete -.separation of a gas having a highdiffusiontrate from'sa :gas having a low diffusion rate, as willbelrliscussedinconnection with Fig. 2, the exhaust pumpshouldrmaintainthepressure of its .associated storage tankiataclevel'ssuitable' .for introduction into the next stages. The pressureshould of. course be suflicient to stress the sphere 14 of the nextstage of a tensile stress in excess of fifty percent of its breakingtensile stress.

The .portionrof. the arrangement of Fig. 1 comprising enclosure 12,-;tube .20, and exhaust pump and storage tanki-116rmay be :convenientlyreferred to as a collecting section or simply as a collector.

For: the above-mentioned specific mixture of helium andhydrogen, at roomtemperature, substantially all of thehelium willditfuse through sphere14 and be collected in section'16, while substantially all of thehydrogen will""remain-in sphere =14and source 10. A one stagediffuser-collector arrangement as illustrated in Fig. 1 will,*therefore,suffice to separate or purify helium from hydrogen atroom temperature.

Iii-the event that one of the gases to be filtered out by thearrangement of Fig. 1 still appears in objectionable quantity inthe'collector section 16, several stages may'be employed in successionas illustrated diagrammatically by'Fig. 2.

In Fig. 2 stages 1, 2 and 3 are represented diagrammaticallyby boxes 40,44 and 48, respectively, and each'may"be-substantially identical to thearrangement illustrated in Fig. 1, except that the storage tank of stage1 (40) is--connected-through a conduit 42 to serve as the--input mixturesource of stage 2, and that of stage 2 (44) is gconnected'through aconduit 46 to serve as the input mixturesource of stage 3 (48).

By way of.illustrative example of the ,efiect of seletitivhdilfit'sioniinvolvingtheuse of several consecutiv'e.difiusencoHeLctor-stages, let.it be assumed that an equal, or one to one, mixture of two gases A and Bis introduced into the source of stage 1 under pressure sufficient tostress the glass diffusing sphere of that stage to a tensile stress inexcess of fifty percent of but less than its breaking tensile stress.Let it be further assumed that the rate of difiusion of gas A throughthe glass sphere is substantially ten times that of gas B. When agivenquantity of gas A- has difiusedthrough -the sphere, only one-tenthas much of gas B will-have beendiffused. Therefore in the collector. ofstage 1 the-mixture will compriseten parts'ofgas A-to 'onerpart of gasB.

In similar manner instage 2, when a given quantity of gas A has diffusedthrough the sphere of that stage, only one-tenth of one-tenthof the..gasB'n/ill have been diffused. In other words, the mixture willcomprise one hundred parts of gas A to one part of gas B. Likewise, atthe output of stage 3 .when a givenquantity vof-. gas;;Ahasdiffusedthroughrthe sphere .of stage 3, there willbea; found to beone thousand parts of gas A to vone.:part:m. gas-B. Additional stagescanobviously be employed iii a further reduction in the amount of gasBpresent desired. Thussubstantially anydesired degree. of purificationcan be realized by the use of a sulficientnumbu; of stages. Two gaseswhich .will behave in:-substan-... tially this manner are heliumandoxygen when..a..dif.-.. fusing medium of unsintered. amorphous:silicaglasszs (thirsty glass) .is employed ineach stage.

Numerous mixtures ofgases canzbe .purifiedz'by the; application of theprinciples of the invention as ;will.be apparent to those skillediin the'art.

In addition to :the 'purificationof helium by. difiusion' from amixtureincluding hydrogen, as a furthen-spe-. cific illustration, hydrogen maybe. diffused from: a tmix+ ture of hydrogen and deuterium usingamorphous silicaz glass, known-by the trade name Vycor, at.a:temperature in the order of eight hundred degrees centigradeaa: therelative ratesv of diffusion being inthe ratio ,ofstwo's to'one,.respectively,.for hydrogen and deuterium. Other specificillustrative examples are hydrogen andv tritium; and neon and helium.

Numerous andvaried other methods and arrangements. within the spirit andscope of the principles of the pres ent invention will readily occur tothose skilled in thee art. The above examples are illustrative of theapplication of said principles but by no meansexhaustively demonstratetheir application.

Whatis claimed is:

1. Apparatus for enhancing the separation or purific'a-a. tion of gasesby diffusion on the basis of atomic-0r molecular size comprising avessel of a vitric material which will pass a predetermined gas bydiffusion but be" substantially impermeable to one or more other gases,means for injecting a mixture of said predetermined gas and said one ormore other gases into said vessel, means 1: to subject the walls of saidvessel to tensile stress in-= excess of fifty percent of the breakingtensile stress of" the material but less than said breaking stress, andmeans to collect gas diffusing through the vessel.

2. A method of separating or purifying gases, comprising injecting themixture of gases to be separated into an enclosure of a vitric materialwhich will pass a predetermined gas of said mixture by diffusion but beimpermeable to other gases of said mixture, increasing the" pressure ofsaid mixture of gases to stress the walls of I the enclosure to atensile stress in excess of fifty percentof its breaking tensilestressbut less than said breaking stress, and collecting the gas whichdiifuscs through said: enclosure.

3. Apparatus for separating a specific gas from a mixture of gases whichcomprises a sphere of a'gla'ss having a diffusing rate for said specificgas substantially, greater-than thediffusing rate for anyother gasjnsaid i mixture, means, for injecting, said mixture ofgases into".

said sphere with a pressure sufficient to subject said sphere to atensile stress in excess of fifty percent of but less than the breakingtensile stress of said glass, and means for collecting the gas whichdiffuses through said sphere.

4. Apparatus for separating helium from hydrogen in a mixture of saidgases, comprising an enclosure of chemical Pyrex glass, means forinjecting said mixture into said enclosure, means for increasing thepressure of said mixture to stress said enclosure to tensile stresses inexcess of fifty percent of but less than the breaking tensile stress ofsaid glass, and means to collect helium diffusing through saidenclosure.

5. A medium for filtering gases of varying diffusion rates, comprising amembrane of a vitrie material which passes predetermined ones of saidgases by differing diffusion rates, respectively, and means for placingsaid membrane under tensile stress exceeding fifty percent of thebreaking tensile stress of said membrane but less than said breakingstress.

References Cited in the file of this patent UNITED STATES PATENTS2,540,151 Weller et al. Feb. 6, 1951 2,734,592 Jones Feb. 14, 19562,773,561 Hunter Dec. 11, 1956

